Conventional optical navigation devices, such as an optical mouse device for a computer, use a light emitting diode (LED) as an illumination source. More recently, a coherent light source, such as a vertical cavity surface emitting laser (VCSEL), is used as the illumination source to increase tracking sensitivity. A laser-based optical navigation device is able to reflect more detail from minor surface defects and textures on tracking surfaces than LED-based optical navigation devices. Using a laser optical navigation device can result in significantly increased tracking sensitivity compared to typical LED optical navigation devices.
FIG. 1 depicts a conventional optical tracking system 10. The conventional optical tracking system 10 includes a laser mouse 12 and a tracking surface 14. The laser mouse 12 includes a laser driver 16, a light source 18, an illumination lens 20, an imaging lens 22, and a sensor array 24. The laser driver 16 generates a fixed output current or a variable output current to drive the light source 18. The variable output current can be adjusted via mechanical adjustments. The illumination lens 20 directs the emitted light towards the tracking surface 14. The imaging lens 22 directs the light reflected off the tracking surface towards the sensor array 24. The sensor array 24 detects the light directed through the imaging lens 22. By capturing a plurality of images of the illuminated tracking surface 14, the captured images are processed and examined to determine movement of the laser mouse 12 relative to the tracking surface 14.
Typically, a coherent light source is biased at some predetermined drive current level to achieve a desired light output level, or output beam power, offering optimum tracking sensitivity. Conventional laser drivers are designed with a fixed resistor or mechanical POT (potentiometer) to control the level of bias current supplied to the coherent light source. To a certain point, as the output beam power increases, so does the tracking sensitivity. However, using a laser in an optical navigation device poses hazards with respect to damage to vision.
The possibility for vision damage is made more uncertain, and potentially more dangerous, by the effects of external influences which may alter the laser output characteristics, in addition to the inherent minor differences in the characteristics of individual coherent light sources. Thus, coherent light sources emit light beams of different output beam power, even when the same current value is driving each coherent light source, making it difficult to ensure operation of lasers below a common eye-safety threshold.